Natural gas is often unavailable in regions where consumers are located, making it necessary to move the natural gas from remote areas. Currently, there are four (4) methods for moving the natural gas between locations: transport by pipeline, liquefaction of the light hydrocarbon, conversion of natural gas to a liquid or solid product to allow for transport, and conversion of natural gas to electricity for transport by cable. Each of these methods has its limitations.
Transport by pipeline is a highly popular method for transport. However, this may not be feasible due to the extreme distances between natural gas resources and consumers, which increases cost.
Liquefaction of the light hydrocarbon allows for several different installations and transport. Baseload plants can produce liquefaction of the light hydrocarbon, but are not commonly found. Currently, baseload plants are available at about fifteen (15) sites throughout the world. Each site has at least one train, and each train can carry up to five (5) million tons per year. Methane tankers are another option for transport. Methane tankers can transport a cryogenic liquid at temperatures of about −160° C., but only about one hundred tankers have this capability. Another possibility for liquefaction of the light hydrocarbon is the LNG terminal. At a LNG terminal, the liquefied natural gas from the methane tanker is unloaded, then vaporized and sent to pipelines. A final option for liquefaction is peak-shaving plants. These small liquefaction plants near consumer zones liquefy and store the natural gas when demand is low and vaporize the gas when demand is high.
Converting the natural gas to liquid or solid products, which may easily be transported, is another possibility. The conversion can be done through several methods. The first method, requires that the natural gas be converted to heavy synthetic hydrocarbons in two stages. With the first stage, synthesis gas, an oxygen enriched gas is required to produce a mixture of hydrogen and carbon monoxide by partial oxidation or autothermal reforming. The second stage requires a catalytic reaction, such as the Fischer-Tropsch type. With the second method for converting the natural gas into a liquid or solid product, natural gas is converted into a methanol or used to produce ammonia or fertilizer.
Finally, natural gas can be converted into electricity in cogeneration plants. The electricity is then transported by cable. Similar to transport by pipeline, this is not economical over long distances.
Liquefaction or conversion of the natural gas both require significant investment to make the process profitable. The first synergy between the two processes (liquefaction and conversion) is to be found in the upstream and downstream infrastructures. Upstream if the two units are on the same site, they may use the same gas fields and the same pipeline to transport natural gas to the site. The pretreatment of the natural gas before liquefaction or transformation into synthesis gas can also be common to the two units. The downstream port infrastructures can also be common. The same utilities (water, steam, instrument air) can be common to the two units.
It has been proposed in WO00/71951 to use the energy produced by the vaporization of liquid nitrogen, liquid oxygen or liquid argon to liquefy natural gas. U.S. Pat. No. 5,390,499 and French Patent 2,122,307 concern heat transfer between vaporising liquid nitrogen and liquefying natural gas. UK Patent 2,172,388 describes an air separation unit which produces oxygen and liquid nitrogen. The liquid nitrogen removed from the air separation unit is then transported to a remote site and used to liquefy natural gas. The gaseous nitrogen produced is then used for enhanced oil recovery.
Regarding liquefaction cycles for the production of LNG, several solutions are described in various publications (for example, “Developments in natural gas liquefaction” in Hydrocarbon Processing April 1999). The most efficient is the cascade refrigeration cycle: refrigeration is provided by three different refrigerants, typically methane, ethylene and propane, each been vaporised at several pressure levels. The most used is the mixed refrigerant cycle with propane precooling where a multicomponent mixture of hydrocarbons (typically propane, ethane, methane and/or nitrogen) perform the final cooling of natural gas while a separate propane cycle perform the precooling of natural gas and mixed refrigerant. This cycle is described in U.S. Pat. No. 3,763,658. The last cycle which has never been used in a baseload plant due to its relative high power consumption is the expander cycle. U.S. Pat. No. 5,768,912 shows various possible improvements of such a cycle but none is able to attain the efficiency of the propane precooled mixed refrigerant cycle.